It was perhaps the greatest scientific achievement of the 20th century. And next week space scientists will celebrate the 100th anniversary of the publication of Albert Einstein’s theory of general relativity in fitting style – by launching a probe to help demonstrate the accuracy of the theory’s last unproven prediction: the existence of gravitational waves.

At 4.15am on 2 December, the satellite, known as Lisa Pathfinder, is scheduled to be blasted into orbit from the European Space Agency’s centre in Kourou, French Guiana. It will carry equipment that will be tested as components for a future orbiting gravitational wave observatory.

“The theory of general relativity is the scientific equivalent of Michelangelo’s Sistine Chapel,” said Pedro Ferreira, professor of astrophysics at Oxford University. “Both are unique works of genius and each could only have been done by one individual. And it is quite stunning that the Lisa Pathfinder satellite – which is designed to help find gravitational waves whose existence is predicted by the theory – is going to be launched on the exact anniversary of the publication of Einstein’s work.”

Gravitational waves are thought to be hurled across space when stars start throwing their weight around, for example, when they collapse into black holes or when pairs of super-dense neutron stars start to spin closer and closer to each other. These processes put massive strains on the fabric of space-time, pushing and stretching it so that ripples of gravitational energy radiate across the universe. These are gravitational waves.

Observations by US astronomers Joseph Taylor and Russell Hulse in the 1980s provided key supporting evidence of their existence. The pair showed that a neutron star, now known as the Hulse-Taylor pulsar, was part of a binary system whose orbit was decaying at a rate consistent with it pumping out gravitational waves. This work won Hulse and Taylor the 1993 Nobel prize in physics. Since then, physicists have tried to spot gravitational waves directly, using ground-based devices with a common design: two long arms, set at right angles to each other, extending from a central point.

When a gravitational wave strikes, it should temporarily shrink one arm and slightly extend the other. That change can then be measured – albeit with considerable difficulty, because any change induced in an arm’s length by a gravitational wave will only be a few hundred billion-billionths of a metre.

So far, researchers have yet to detect such changes. Once they do, one of the final hurdles to a complete understanding of the makeup of our universe will have been achieved. However, they are now extending their efforts to space because, in orbit, it should be possible to fly detectors that are 5 million kilometres apart and which will be better able to spot the compressing and stretching of space-time.

“Over these huge distances, the effect of a gravitational wave is much greater and so it becomes much easier to detect one,” said Paul McNamara, the mission’s project scientist.

The Pathfinder probe will test equipment that will later be used to build a full-scale orbiting gravitational wave detector, a device that will be known as Lisa, the Laser Interferometer Space Antenna. However, astronomers will have to wait some time before it arrives because the European Space Agency does not plan to launch Lisa until 2034.

“It is a simply a matter of budgets. There are so many other great space projects competing for money,” said McNamara. “Nevertheless, a space-based gravitational wave observatory will revolutionise astronomy. We are not merely building a detector. We will have a machine that will use gravitational waves to study the universe.

“It will be an observatory and it will use a completely new medium to peer into the universe’s dark corners. Of course, a great many technological hurdles will have to be overcome. This is a very ambitious project. Nevertheless, we believe we can do it.”

At the heart of Lisa Pathfinder are two 2kg cubes of gold and platinum. These will be allowed to float free inside the craft, while being shielded from all forms of radiation and particle bombardment. The task of Lisa Pathfinder will then be to determine if it is possible to use lasers to measure deviations in their movements inside the craft with an accuracy of a trillionth of a metre. “We have worked very hard on this and we are sure that we can do it,” added McNamara.

“This is a great mission and the fact that it is going to be launched exactly 100 years after the publication about general relativity on 2 December, 1915, just makes the experience all the richer for us.”

THEORY THAT WAS YEARS IN THE MAKING

In his On the General Theory of Relativity, Albert Einstein took previous ideas about gravity – which had been seen as a force acting between objects – and replaced them with a description of it as being a geometric property of space and time.

Many predictions emanated from this idea: the existence of super-dense collapsed stars – now known as black holes – gravitational lensing, in which light from a distant source is distorted by a huge, intermediary object, gravitational waves and many others.

“All of these phenomena have now been observed – with one exception, and that is gravitational waves,” said Pedro Ferreira, whose book The Perfect Theory, published last year, outlined the struggles that Einstein endured in working out his theory.

“It was like a piece of art, the effort of one man, who struggled for seven years just to get things right,” said Ferreira. “He worked on his ideas and equations exhaustively and then presented his theory in a series of lectures in November 1915 culminating in a final presentation on the 24th. Then he published in an unbelievably short time on 2 December.”

At the time the publication of Einstein’s theory attracted little attention. Europe was then convulsed by war. However, the theory made global headlines shortly after the war, when the British astronomer Arthur Eddington studied star positions during a total solar eclipse and found their light was being bent by the Sun in a manner consistent with Einstein’s theory.

However, it took the development of radio astronomy to fully underline the importance of general relativity. “Astronomers began to detect ultra-energetic, incredibly dense objects that could only be understood in terms of their gravitational fields,” said Ferreira. “Gravity had to be really, really important, and the only theory that makes it possible for us to understand gravity is general relativity.”